1. Technical Field
The invention relates to a liquid methylene bis(phenylisocyanate) composition having improved storage stability.
2. Description of Prior Art
Polyisocyanate compositions including a high concentration of methylene bis(phenylisocyanate) ("MDI"), particularly 4,4'-methylene bis(phenylisocyanate) ("4,4'-MDI"), while useful for various cellular and non-cellular polyurethane applications often pose a processing problem in that they are normally a solid at room temperature, i.e., about 25.degree. C. The material, therefore, has to be melted and maintained in order to be useful as a liquid.
Unfortunately, methylene bis(phenylisocyanate) compositions having relatively high levels of 4,4'-MDI are also known to have a limited life due to the formation of diphenylmethane uretdione, otherwise referred to herein as uretdione. Uretdione tends to couple with the 4,4'-MDI molecules contained in methylene bis(phenylisocyanate) compositions, thereby forming a substantially insoluble precipitate over time. For example, 4,4'-MDI compositions maintained at about 43.degree. C. for 14 days have exhibited uretdione concentrations above the generally acceptable saturation concentration of 0.5%. The formation of high concentrations of uretdione have heretofore rendered the methylene bis(phenylisocyanate) compositions substantially useless.
Interestingly, the uretdione reaction is both temperature and phase dependent. For example, as the temperature of a 4,4'-MDI composition is increased above about 43.degree. C., the rate of uretdione formation increases. Further, the rate of uretdione formation is accelerated when the methylene bis(phenylisocyanate) is in the solid state as compared to a liquid composition at 43.degree. C.; such formation being generally attributed to the alignment of the isocyanate groups in the crystal lattice structure.
In an effort to limit the formation of uretdione, methylene bis(phenylisocyanate) compositions, particularly 4,4'-MDI compositions, are often frozen and stored at temperatures below about 0.degree. C. While this temporarily retards the formation of uretdione dimer, refrigerating the compositions for extended periods of time prior to use can be expensive and introduces logistic considerations which must be addressed.
It is, therefore, an object of the present invention to provide a liquid methylene bis(phenylisocyanate) composition which is storage stable at temperatures above about 30.degree. C.
It is a further object of the invention to provide a liquid methylene bis(phenylisocyanate) composition which increases the saturation concentration of uretdione.
It is another object of the present invention to provide a methylene bis(phenylisocyanate) having a reduced freezing point.
It is still another object of the present invention to provide a liquid methylene bis(phenylisocyanate) composition which can be used in the preparation of all types of polyurethanes for which pure MDI is currently employed.
In view of the foregoing, the development of a liquid storage stable methylene bis(phenylisocyanate) composition including a high concentration of 4,4'-MDI that could be shipped in bulk, thereby reducing the high costs associated with shipping frozen drummed compositions would be highly desirable.